Aluminum and alloys of aluminum are chemically machined, etched, in solutions of alkali hydroxides, especially sodium hydroxide. The machining rate decreases with machining and it is necessary to periodically remove the aluminum from the sodium hydroxide or to discard the etchant solution. It would be preferable if the composition of the etchant could be maintained constant at an aluminum content optimum for machining and substantially all of the sodium hydroxide could be continuously used as the etchant. It is possible to remove aluminum hydroxide or sodium aluminate from the sodium hydroxide by cooling the solution, seeding and crystallizing with removal by filtration. This process is, however, difficult to carry out, limited in the percentage of aluminum that can be removed from the etchant and requires a relatively large investment in crystallization and filtration equipment. It is an object of the instant invention to provide a process for removing aluminum from the sodium hydroxide etchant that is easy to operate, permits continuous removal of aluminum and reuse of sodium hydroxide and essentially zero discharge of liquid from the etchant process.
Many of the hydroxides of heavy metals such as aluminum, lead, tin, zinc, gallium and tungsten are soluble or appear to be soluble in excess of sodium or potassium hydroxide. This has been commonly attributed to the formation of salts of the hydroxides behaving as amphoteric substances (acid or basic properties) and giving either OH or H ions according to the conditions of the experiment. In the presence of strong alkalis they are supposed to behave as acids. For example when aluminum hydroxide is dissolved in sodium hydroxide sodium aluminate is supposed to be formed. It is possible, however, that the solution of the hydroxide is not so much a matter of compound formation as of peptization (change into a colloid or gel) of the hydroxide to form a sol. Aluminum is usually or etched in a 30 wt% sodium hydroxide solution at about 80.degree. C. until 30 or more grams of aluminum per liter has been etched. The aluminum appears to be dissolved and it would be expected that concentration of the aluminum in the etchant by removal of the sodium hydroxide would, at some concentration, result in the precipitation of the aluminum as aluminum hydroxide or sodium aluminate. I have found, however, that removal of the sodium hydroxide by electrodialysis does not result in insolubilization of the aluminum until essentially all of the sodium hydroxide has been removed. The electrical conductivity of the aluminum-sodium hydroxide solution is equal to that of a plain solution of the sodium hydroxide which indicates that the aluminum is completely hydrolyzed and not ionic. The aluminum does not diffuse through paper or plastic membranes. It appears that the aluminum is colloidal and the hydroxyl ions in the solution is substantially that of the alkali. Although some sodium aluminate may be formed in very concentrated sodium hydroxide solutions, the formation of sodium aluminate in the usual sodium hydroxide etchants is relatively unimportant compared with the formation of the colloidal solution. It is obviously a difficult task to cool, seed and crystallize sodium aluminate or aluminum hydroxide from the colloidal solutions. Electrodialysis is a well known art (see U.S. Pat. Nos. 4,325,792; 4,439,293 and 4,626,288, the disclosures of which are incorporated by reference). Electrodialysis is the transport of ions through ion permeable membranes as a result of an electrical driving force. The process is commonly carried out in an electrochemical cell having a catholyte compartment containing a cathode and a catholyte and an anolyte compartment containing an anode and an anolyte, the catholyte and anolyte compartments being separated by ion permeable membranes. The electrotransport of sodium and other alkali metal cations through cation permeable membranes is a known art. However, prior processes do not provide a satisfactory solution to the problem of precipitation or insolubilization of aluminum hydroxide and other metal acidic oxides in alkaline solutions so that the aluminum hydroxide and other oxides or hydroxides can be separated and removed from the alkaline solution and the alkali hydroxide recovered for use. The prior processes do not provide a means of changing, varying, in electrolysis the pH of the alkaline solution to selectively insolubilize and remove aluminum and other heavy metal acidic oxides from alkaline solutions.